Nuclear reactors are used in electric power generation, research and propulsion. A reactor pressure vessel contains the reactor coolant, i.e. water, which removes heat from the nuclear core. Respective piping circuits carry the heated water or steam to the steam generators or turbines and carry circulated water or feedwater back to the vessel. Operating pressures and temperatures for the reactor pressure vessel are about 7 MPa and 288.degree. C. for a boiling water reactor (BWR), and about 15 MPa and 320.degree. C. for a pressurized water reactor (PWR). The materials used in both BWRs and PWRs must withstand various loading, environmental and radiation conditions.
Some of the materials exposed to high-temperature water include carbon steel, alloy steel, stainless steel, and nickel-based, cobalt-based and zirconium-based alloys. Despite careful selection and treatment of these materials for use in water reactors, corrosion occurs on the materials exposed to the high-temperature water. Such corrosion contributes to a variety of problems, e.g., stress corrosion cracking, crevice corrosion, erosion corrosion, sticking of pressure relief valves and buildup of the gamma radiation-emitting Co-60 isotope.
Stress corrosion cracking (SCC) is a known phenomenon occurring in reactor components, such as structural members, piping, fasteners, and welds, exposed to high-temperature water. As used herein, SCC refers to cracking propagated by static or dynamic tensile stressing in combination with corrosion at the crack tip. The reactor components are subject to a variety of stresses associated with, e.g., differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stress from welding, cold working and other asymmetric metal treatments. In addition, water chemistry, welding, crevice geometry, heat treatment, and radiation can increase the susceptibility of the metal in a component to SCC.
It is well known that SCC occurs at higher rates when oxygen is present in the reactor water in concentrations of about 1 to 5 parts per billion (ppb) or greater. SCC is further increased in a high radiation flux where oxidizing species, such as oxygen, hydrogen peroxide, and short-lived radicals, are produced from radiolytic decomposition of the reactor water. Such oxidizing species increase the electrochemical corrosion potential (ECP) of metals. Electrochemical corrosion is caused by a flow of electrons from anodic to cathodic areas on metallic surfaces. The ECP is a measure of the thermodynamic tendency for corrosion phenomena to occur, and is a fundamental parameter in determining rates of, e.g., SCC, corrosion fatigue, corrosion film thickening, and general corrosion.
Corrosion potential has been clearly shown to be a primary variable in controlling the susceptibility to SCC in BWR environments. FIG. 1 shows the observed and predicted crack growth rate as a function of corrosion potential for furnace-sensitized Type 304 stainless steel at 27.5 to 30 MPa.sqroot.m in 288.degree. C. water over the range of solution conductivities from 0.1 to 0.5 .mu.S/cm. Data points at elevated corrosion potentials and growth rates correspond to irradiated water chemistry conditions in test or commercial reactors.
Corrosion (or mixed) potential represents a kinetic balance of various oxidation and reduction reactions on a metal surface placed in an electrolyte, and can be decreased by reducing the concentration of oxidants such as dissolved oxygen. FIG. 2 is a schematic of E (potential) vs. log .vertline.i.vertline. (absolute value of current density) curves showing the interaction of H.sub.2 and O.sub.2 on a catalytically active surface such as platinum or palladium. i.sub.0 is the exchange current density, which is a measure of the reversibility of the reaction. Above i.sub.0, activation polarization (Tafel behavior) is shown in the sloped, linear regions. i.sub.L represents the limited current densities for oxygen diffusion to the metal surface, which vary with mass transport rate (e.g., oxygen concentration, temperature, and convection). The corrosion potential in high-temperature water containing oxygen and hydrogen is usually controlled by the intersection of the O.sub.2 reduction curve (O.sub.2 +2H.sub.2 O+4e.sup.- .fwdarw.4OH.sup.-) with the H.sub.2 oxidation curve (H.sub.2 .fwdarw.2H.sup.+ +2e.sup.-), with the low kinetics of metal dissolution generally having only a small role.
The fundamental importance of corrosion potential versus, e.g., the dissolved oxygen concentration per se, is shown in FIG. 3, where the crack growth rate of a Pd-coated CT specimen drops dramatically once excess hydrogen conditions are achieved, despite the presence of a relatively high oxygen concentration. FIG. 2 is a plot of crack length vs. time for a Pd-coated CT specimen of sensitized Type 304 stainless steel showing accelerated crack growth at .apprxeq.0.1 .mu.M H.sub.2 SO.sub.4 in 288.degree. C. water containing about 400 ppb oxygen. Because the CT specimen was Pd-coated, the change to excess hydrogen caused the corrosion potential and crack growth rate to drop.
In a BWR, the radiolysis of the primary water coolant in the reactor core causes the net decomposition of a small fraction of the water to the chemical products H.sub.2, H.sub.2 O.sub.2, O.sub.2 and oxidizing and reducing radicals. For steady-state operating conditions, approximately equilibrium concentrations are established for O.sub.2, H.sub.2 O.sub.2, and H.sub.2 in the water which is recirculated and for O.sub.2 and H.sub.2 in the steam going to the turbine. These concentrations of O.sub.2, H.sub.2 O.sub.2, and H.sub.2 are oxidizing and result in conditions that can promote intergranular stress corrosion cracking (IGSCC) of susceptible materials of construction. One method employed to mitigate IGSCC of susceptible material is the application of hydrogen water chemistry (HWC), whereby the oxidizing nature of the BWR environment is modified to a more reducing condition. This effect is achieved by adding hydrogen gas to the reactor feedwater. When the hydrogen reaches the reactor vessel, it reacts with the radiolytically formed oxidizing species homogeneously and on metal surfaces to reform water, thereby lowering the concentration of dissolved oxidizing species in the water in the vicinity of metal surfaces. The rate of these recombination reactions is dependent on local radiation fields, water flow rates and other variables.
The injected hydrogen reduces the level of oxidizing species in the water, such as dissolved oxygen, and as a result lowers the ECP of metals in the water. However, factors such as variations in water flow rates and the time or intensity of exposure to neutron or gamma radiation result in the production of oxidizing species at different levels in different reactors. Thus, varying amounts of hydrogen have been required to reduce the level of oxidizing species sufficiently to maintain the ECP below a critical potential required for protection from IGSCC in high-temperature water. As used herein, the term "critical potential" means a corrosion potential at or below a range of values of about -0.230 to -0.300 V based on the standard hydrogen electrode (SHE) scale. IGSCC proceeds at an accelerated rate in systems in which the ECP is above the critical potential, and at a substantially lower or zero rate in systems in which the ECP is below the critical potential (see FIG. 1). Water containing oxidizing species such as oxygen increases the ECP of metals exposed to the water above the critical potential, whereas water with little or no oxidizing species present results in an ECP below the critical potential.
Initial approaches to reduce the corrosion potential focused on relatively large additions of dissolved hydrogen, which proved capable of reducing the dissolved oxygen concentration in the water outside of the core from .apprxeq.200 ppb to &lt;5 ppb, with a resulting change in corrosion potential from .apprxeq.+0.05 V.sub.she to .ltoreq.-0.25 V.sub.she. This approach is termed hydrogen water chemistry, and is in commercial use in domestic and overseas BWRs. Corrosion potentials of stainless steels and other structural materials in contact with reactor water containing oxidizing species can usually be reduced below the critical potential by injection of hydrogen into the feedwater. For adequate feedwater hydrogen addition rates, conditions necessary to inhibit IGSCC can be established in certain locations of the reactor. Different locations in the reactor system require different levels of hydrogen addition. Much higher hydrogen injection levels are necessary to reduce the ECP within the high radiation flux of the reactor core, or when oxidizing cationic impurities, e.g., cupric ion, are present.
It has been shown that IGSCC of Type 304 stainless steel (containing 18-20% Cr, 8-10.5% Ni and 2% Mn) and all other structural materials used in BWRs can be mitigated by reducing the ECP of the stainless steel to values below -0.230 V(SHE). An effective method of achieving this objective is to use HWC. However, high hydrogen additions, e.g., of about 200 ppb or greater in the reactor water, that may be required to reduce the ECP below the critical potential, can result in a higher radiation level in the steam-driven turbine section from incorporation of the short-lived N.sup.16 species in the steam. For most BWRs, the amount of hydrogen addition required to provide mitigation of IGSCC of pressure vessel internal components results in an increase in the main steam line radiation monitor by a factor of five to eight. This increase in main steam line radiation can cause high, even unacceptable, environmental dose rates that can require expensive investments in shielding and radiation exposure control. Thus, recent investigations have focused on using minimum levels of hydrogen to achieve the benefits of HWC with minimum increase in the main steam radiation dose rates.
An effective approach to achieve this goal is to either coat or alloy the stainless steel surface with palladium or other noble metals. The presence of palladium on the stainless steel surface reduces the hydrogen demand to reach the required IGSCC critical potential of -0.230 V(SHE). The use of alloys or metal coatings containing noble metals permits lower corrosion potentials (e.g., .apprxeq.-0.5 V.sub.she) to be achieved at much lower hydrogen addition rates. For example, U.S. Pat. No. 5,135,709 to Andresen et al. discloses a method for lowering the ECP on components formed from carbon steel, alloy steel, stainless steel, nickel-based alloys or cobalt-based alloys which are exposed to high-temperature water by forming the component to have a catalytic layer of a noble metal. Such approaches rely on the very efficient recombination kinetics of dissolved oxygen and hydrogen on catalytic surfaces (see the high i.sub.O for H.sub.2 oxidation in FIG. 2, which causes most O.sub.2 reduction curves to intersect at -0.5 V.sub.she). This was demonstrated not only for pure noble metals and coatings, but also for very dilute alloys or metal coatings containing, e.g., &lt;0.1 wt. % Pt or Pd (see FIGS. 3 to 5). FIG. 4 shows corrosion potential measurements on pure platinum, Type 304 stainless steel and Type 304 stainless steel thermally sprayed by the hyper velocity oxy-fuel (HVOF) technique with a powder of Type 308L stainless steel containing 0.1 wt. % palladium. Data were obtained in 285.degree. C. water containing 200 ppb oxygen and varying amounts of hydrogen. The potential drops dramatically to its thermodynamic limit of .apprxeq.-0.5 V.sub.she once the hydrogen is near or above the stoichiometric value associated with recombination with oxygen to form water (2H.sub.2 +O.sub.2 .fwdarw.2H.sub.2 O) . FIG. 5 shows corrosion potentials of Type 304 stainless steel doped with 0.35 wt. % palladium at a flow rate of 200 cc/min. in 288.degree. C. water containing up to 5000 ppb oxygen and various amounts of hydrogen.
If the surface recombination rate is much higher than the rate of supply of oxidants to the metal surface (through the stagnant, near-surface boundary layer of water), then the concentration of oxidants (at the surface) becomes very low and the corrosion potential drops to its thermodynamic limit of .apprxeq.-0.5 V.sub.she in 288.degree. C. water, even though the bulk concentration of dissolved oxygen remains high (FIGS. 3 to 5). Further, the somewhat higher diffusion rate of dissolved hydrogen versus dissolved oxygen through the boundary layer of water permits somewhat substoichiometric bulk concentrations of hydrogen to support full recombination of the oxidant which arrives at the metal surface. While some hydrogen addition to BWRs will still be necessary with this approach, the addition can be vastly lower than (as low as .ltoreq.1% of) that required for the initial hydrogen water chemistry concept. Hydrogen additions remain necessary since, while oxidants (primarily oxygen and hydrogen peroxide) and reductants (primarily hydrogen) are produced by radiolysis in stoichiometric balance, hydrogen preferentially partitions to the steam phase in a BWR. Also, no hydrogen peroxide goes into the steam. Thus, in BWR recirculation water there is some excess of oxygen relative to hydrogen, and then, in addition, a fairly large concentration of hydrogen peroxide (e.g., .apprxeq.200 ppb). Approaches designed to catalytically decompose the hydrogen peroxide before or during steam separation (above the core) have also been identified.
While the noble metal approach works very well under many conditions, both laboratory data and in-core measurements on noble metals show that it is possible for the oxidant supply rate to the metal surface to approach and/or exceed the recombination rate (see FIGS. 6 and 7). FIG. 6 shows the effect of feedwater hydrogen addition on the corrosion potential of stainless steel and platinum at several locations at the Duane Arnold BWR. At .apprxeq.2 SCFM of feedwater hydrogen addition, the corrosion potentials in the recirculation piping drop below .apprxeq.-0.25 V.sub.she . However, in the high flux (top of core) regions, even for pure Pt, the corrosion potential remains above .apprxeq.-0.25 V.sub.she at feedwater hydrogen levels of .gtoreq.15 scfm, where long-term operation is very unattractive due to the cost of hydrogen and the increase in volatile N.sup.16 (turbine shine). FIG. 7 shows corrosion potential vs. hydrogen addition for Pd-coated Type 316 stainless steel in 288.degree. C. water in a rotating cylinder specimen, which simulates high fluid flow rate conditions. The water contained 1.0 parts per million (ppm) O.sub.2. As the hydrogen level was increased above stoichiometry, the potential decreased, but only to about -0.20 V.sub.she. The oxygen supply rate in these tests had exceeded the exchange current density (i.sub.O) of the hydrogen reaction (see FIG. 2), and activation polarization (Tafel response) of the hydrogen reaction began to occur, causing a shift to a mixed (or corrosion) potential which is in between the potentials measured in normal and extreme hydrogen water chemistry on noncatalytic surfaces.
At the point where the oxidant supply rate to the metal surface approaches and/or exceeds the recombination rate, the corrosion potential will rapidly increase by several hundred millivolts (e.g., to .gtoreq.-0.2 V.sub.she). Indeed, even under (relatively small) excess hydrogen conditions, pure platinum electrodes in the core of BWRs exhibit corrosion potentials which are quite high, although still somewhat lower than (noncatalytic) stainless steel (see FIG. 6). At very high hydrogen levels (well above those typically used in the original hydrogen water chemistry concept), the corrosion potential on noble metal surfaces will drop to &lt;-0.3 V.sub.she (see FIG. 6). However, the huge cost of the hydrogen additions combined with large observed increase in volatile radioactive nitrogen in the steam (i.e., N.sup.16, which can raise the radiation levels in the turbine building) make the use of very high hydrogen addition rates unpalatable.